Overcurrent conditions in a DC-to-DC converter circuit are normally sensed by directly monitoring a current to be controlled, and initiating a current limit operation when it reaches a predetermined threshold level. This technique normally requires generating a current-responsive signal from some sensing device in the current path, and transmitting it to some control circuit, which evaluates it and initiates an appropriate control response. If transmission of the current-responsive signal is from the secondary to the primary side of the power circuit, separate signal isolation means may be required between the primary and secondary circuits.
Current may be sensed in the flyback path of the converters output filter rather than the main output power path of a converter. The short duration of the current flow therein means that the average current flow through the current-sensing device per cycle of operation is very small; and, hence, the power dissipation incurred in the sensing device is very low. Such an arrangement is disclosed in my U.S. Pat. No. 4,058,758, issued Nov. 15, 1977, and assigned to the same assignee as this application. In that arrangement, current in the flyback path is sensed by a current-sensing resistor and amplified and coupled through an optoisolator to a duty cycle control on the primary side of the converter. The impedance of the current-sensing resistor is kept as low as possible and as far as is consistent with the ability of a sensing amplifier to accurately amplify a voltage drop across it into a useful level voltage signal to be used in the current control process. The typical low voltage obtained requires a very stable and accurate amplifier, and other circuit components which usually make up a significant portion of the cost of the converter.